It is known to secure the shank of such a tool in an annular, intrinsically closed tool holder. For this purpose, in its tool receiving opening, the sleeve part has a mating surface, which is concentric to the rotation axis of the tool holder and to which the shank of the tool can be fastened in a press fit.
The sleeve part of the tool holder can be expanded with heat in the radial direction until the cold shank of the tool can be brought into or out of engagement with the mating surface of the sleeve part, i.e. can be inserted into the sleeve part or removed from it. As soon as the sleeve part has cooled again, a press-fit connection is established between it and the tool shank, which is slightly larger than the mating surface of the sleeve part. This press-fit connection reliably secures the tool in the tool holder (shrink-fit technology).
Tool holders whose sleeve part is embodied with thick walls are particularly used in practical applications with special requirements regarding rigidity. Constant problems arise when unclamping tools from tool holders of this kind.
In order to be able to unclamp the tool again at a particular time, it is common practice to use an inductive heating device, i.e. a high-frequency induction coil that induces eddy currents at the outer circumference surface of the sleeve part, which heat the sleeve part from the outside. As this occurs, the shank of the tool to be unclamped is shielded so that the shank itself is not heated. Particularly at high frequencies, the so-called skin effect occurs; in other words, the heat-generating eddy currents are actually induced only a short distance beneath the surface of the sleeve part. The heat thus generated beneath the surface of the outer circumference surface of the sleeve part does penetrate in the direction of the middle of the sleeve through heat conduction, but only in a delayed fashion. At a particular point in time, the heat introduced into the sleeve part has caused it to expand until the shank of the tool, which is still cold at that point, can be removed from the sleeve part because the previously existing press fit has been released due to the increase in the diameter of the sleeve part.
With increasing thickness of the sleeve part in the radial direction, however, problems arise in this regard. The thicker the sleeve part is in the radial direction, the more unevenly it is heated—at some point, a stage arrives in which the sleeve part has already been heated fairly heavily in the region of the outermost radii while it remains relatively cold in the region of the innermost radii. This results in relatively powerful thermal stresses. The outer region of the sleeve part that is not only expanding in the radial direction, but also elongating a significant amount in the axial direction demonstrates a perceptible tendency to bend the sleeve part inward once its elongation in the axial direction is hindered too forcefully by the still cold inner region. This looks (schematically depicted) approximately like the depiction in FIG. 1, in which it is shown in very exaggerated fashion for the sake of better visibility.
This unwanted deformation of the sleeve part is counterproductive since as a result of it, the tool shank initially continues to be locally clamped by the mating surface even though the sleeve part has already expanded far enough that a large part of the mating surface has disengaged from the tool shank. At the very least, this reduces the window of time during which the tool shank can be smoothly withdrawn from the sleeve part. This is not desirable. In addition, forcing the sleeve part to expand far enough for its mating surface to truly disengage from the tool shank at all locations requires the transmission of an unnecessarily large amount of heat to the sleeve part. This is also undesirable.
In order to avoid these problems, it is common practice to keep the wall thicknesses of the sleeve parts to the minimum possible. For this reason, according to DIN 69882-8, standard shrink-fit chucks have the following wall thicknesses at their tips (measured in mm):
Clamping diameterWall thickness 67.5 86.5107.0126.0146.5165.5187.5206.5259.5326.0
German patent DE 102 44 759 has already attempted to solve the problem of the unwanted deformation of the sleeve part. In order to prevent the unwanted deformation, this patent proposes sinking a deeply incised radial annular groove into the mating surface all the way in front, in the region of the tip of the sleeve part, which groove delimits the front region of the actual mating surface. This is based on the idea that the thus weakened region at the tip of the sleeve part is able to expand unhindered in the radial direction, consequently preventing the sleeve part from bending inward too much in the region of its tip. Therefore, DE 102 44 759 can be said to be generally based on the concept of counteracting the unwanted inward bending of the tip region of the sleeve part by overlaying this with an opposite effect.
The cited patent also proposes delimiting the mating surface by means of an additional annular groove at its inward end oriented away from the tool. The actual mating surface, however, which the cited patent refers to as the “main clamping surface 13,” is not interrupted by annular recesses in the embodiment proposed according to the cited patent, but rather, merely bordered by the annular grooves at both ends.
Other remedies proposed by the cited patent include, if necessary, providing additional grooves extending in the axial direction, which divide the mating surface or main clamping surface into individual elastic segments that each constitute a kind of spring leaf and are situated next to one another in the circumference direction.
The measures proposed by the cited patent, however, are insufficient, particularly in tool holders with thick-walled sleeve parts and—if the axial grooves proposed by the cited patent are also used for controlling the problem of sleeve part warpage—result in a loss of precision and also weaken the press fit. In addition, the greatest forces (bending moments and transverse forces from the tool) are transmitted right in the front region of the sleeve part. A weakening of the sleeve part in this region therefore results in a sharp reduction in rigidity even if no axial grooves are used and should therefore be avoided.
The German patent application DE 10 2004 042 770 also describes a tool holder with a sleeve part into which the shank of a tool can be shrink-fitted. The mating surface, which is provided for this purpose on the inside of the sleeve part of this tool holder, is interrupted by a series of narrowly spaced recesses and is divided into individual, very narrow segments. The reason for this division of the mating surface into individual, small segments is to turn the small segments into barbs of a sort, which provide an increased resistance to an unwanted removal of the tool shank in the axial direction. This is because the small segments are, generally speaking, conceived so that due to static friction, they are carried along by the tool shank when the latter is withdrawn a short distance in the axial direction. The flexibility of these small segments causes them to lift up, thus increasing the pressure between the sleeve part and the tool shank. This therefore achieves a kind of self-locking.
Since the sleeve parts of the tool holders disclosed by the cited patent application clearly have only the conventional wall thickness, the recesses used here do not have any perceptible influence on the expansion behavior of the sleeve part during heating. This is because with thin walls, the still cold inner region is “pulled along” by the already hot outer region, i.e. either way, it is expanded sufficiently to permit unhindered insertion and primarily also subsequent removal of the tool shank.
Naturally, such a design in which the mating surface in the sleeve part is divided up into nothing but small, flexible segments is not particularly rigid.
By contrast, the object of the present invention is to disclose a tool holder whose sleeve part can be embodied with thick walls and permits an extremely rigid clamping of the tool with superior quality centering.